Gas turbines comprise series of circumferentially distributed vanes connected to a cast stator by a shroud with fixation features for fixing the vane to the stator. Each shroud of a vane is suitably sealed against the shroud of adjacent vanes. Sealing adds significant complexity to the assembly and must be considered in vane design in order to optimize the overall efficiency of the gas turbine. It may therefore be preferable to reduce seal length. This can be achieved by, for example, designing the vane with a reduced chord length so by downsizing the shroud resulting in less sealing area and shorter seal length. Changing the vane dimension for reasons other than to improve its aerodynamic performance however, can lead to an overall loss in the vane's efficiency.
As an alternative, vanes may be manufactured as multi vane segments such that each segment comprises a common platform and shroud between which two or more airfoils are formed. In this way, the overall seal length per vane is reduced for a given design enabling the design Engineer more freedom to consider the aerodynamic performance of the vane.
Examples of multi vane segments are disclosed in US 2007/0122266 A1, which teaches of a multi vanes arrangement where airfoils are individually sealed onto a common platform. U.S. Pat. No. 6,435,813 and U.S. Pat. No. 7,377,743 teach of other multi vane arrangements in which the segments may be cast in one piece.
As castings become larger and more complex, by for example taking the form of multi vane segment castings, shrinkage problems increase and it becomes harder to control the casting process parameters. This known problem leads to higher rejections rates. As a result, complex multi vane segment designs, which can economically be cast for small aero engine multi vanes, may have unacceptably high rejection rates when cast as larger industrial gas turbine multi vane segments.